1. Field of the Invention
This invention relates to a phosphorescent synthetic resin material capable of absorbing and accumulating light therein in the light and phosphorescing in the dark, a method for the production of the material, and a formed article or molded article made of the phosphorescent synthetic resin material.
2. Description of the Prior Art
Heretofore, ZnS:Cu, (Ca, Sr)S:Bi, etc. have been known as phosphorescent pigments (long afterglow phosphors). Generally, zinc sulfide has been widely used.
These phosphorescent pigments have the nature of exciting themselves by absorbing the light such as the daylight or the fluorescent light (accumulation of light) and, when placed in a dark place, emitting light. Since these phosphorescent pigments are capable of repeating the cycle of absorbing-accumulating-emitting light over and over again, they find extensive utility in various applications such as marks and signs for prevention of disasters and preservation of safety used in such a place as an emergency exit which should be perceived visually even in the dark, various guide signs used in automobiles and buildings, dials in watches, and such accessories as tie pins and pendants.
For general-purpose applications, these phosphorescent pigments are generally used in the form of phosphorescent sheets or paints by reason of the convenience of handling.
As an example of the incorporation of a phosphorescent pigment in resin, published Japanese Utility Model Application, KOKAI (Early Publication) No. (hereinafter referred to briefly as "JUM-A-") 61-179,911 proposes the formation of engaging elements of a slide fastener with a synthetic resin containing zinc sulfide as a phosphorescent pigment at a concentration in the range of 25 to 30% by weight.
Generally, when a phosphorescent pigment is incorporated in a synthetic resin, however, this incorporation entails the disadvantage that the phosphorescent pigment is not easily dispersed uniformly in the synthetic resin and the resultant phosphorescent synthetic resin inevitably emits light of uneven color.
Since zinc sulfide which has been heretofore used generally as a phosphorescent pigment phosphoresces with unduly low luminance, it is necessary that the zinc sulfide be combined with a radioactive substance and consequently enabled by the radiation of the added substance to be sufficiently excited or, as stated in JUM-A-61-179,922 mentioned above, the phosphorescent pigment be incorporated in the synthetic resin in a large amount.
The radioactive substance, however, has the problem that it must be handled under a rigid standard of safety control and, moreover, the disposal of the waste arising therefrom incurs great expense. Further, when the phosphorescent pigment is incorporated in a large amount in the synthetic resin, the problem ensues that the resultant phosphorescent resin will not easily acquire sufficient strength necessary for a resinous product.